The lever escapement

An escapement lets the wheels of a watch advance one step at each swing of the balance and gives the balance a small push to keep it going.^1, 5

In Mudge’s lever a pivoted lever carries two pallets that lock and release the escape wheel; a fork at its other end meets a pin on the balance just as it swings through the centre.^1, 4

Its great advantage is that it is detached: for most of each swing the balance runs free of the train. It also restarts itself if a jolt stops the balance.^1, 5

The earlier rack lever kept a toothed lever always in mesh with a pinion on the balance; the 1911 Britannica calls the lever 'originally a French invention' that Mudge greatly improved.^4, 5

The date of Mudge’s lever is given as 1750, about 1754 and by 1755; Wikipedia says he used it in a clock before a watch.^2, 5, 7

The Royal Collection calls Queen Charlotte’s watch, signed by Mudge and cased in 1769-70, the earliest known lever watch.^2, 3

Mudge made few: he turned to marine timekeepers, and the escapement spread slowly.^3, 7

Josiah Emery of Charing Cross made lever watches from about 1781, at the start on Mudge’s layout and by 1792 as a straight-line lever without draw.^8, 9, 10

Peter Litherland of Liverpool patented a rack lever in 1791, and Edward Massey a modified lever used by Liverpool makers about 1800.^{1, 11, 12}

Early levers lacked draw, the angle of the pallets that holds the lever safely against its banking pins; some makers thought draw caused friction, but without it a jolt could unlock the escapement.^1, 9

Wikipedia says Breguet improved the lever in 1787, and that George Savage devised the table-roller form in the early 1800s.¹

Two forms followed: the English lever, with the lever at right angles, and the in-line lever later used by Swiss and American makers, which became the modern form; Emery was already making a straight-line lever in London in 1792.^5, 9

A cheap pin-pallet version put the lever into alarm clocks and dollar watches; Wikipedia credits it to Georges-Frédéric Roskopf, mass-produced from 1867, in one article and to Louis Perron in 1798 in another.^1, 5

By the 1911 Britannica it was 'now universally made'; Wikipedia says almost every mechanical watch has used it since about 1900.^1, 4